Conventional sequential injection systems and processes inject polymeric materials one at a time in a predefined sequence to create plastic articles having a five layer structure. One conventional sequential injection process might have three phases, the first injects a first polymeric material, the second injects a second polymeric material, and the third injects a third polymeric material. For this sequential injection process, the quantity of skin material in the first shot determines the location of the leading edge or start of the second material in the final part and the quantity of material in the second shot determines the quantity of second material in the final part. When creating plastic articles on a multi-cavity system it is important to control the polymeric material flow entering each cavity so that each cavity creates a nearly identical article. This is particularly true for the first and second phases of a sequential process.
Simultaneous injection systems inject multiple polymeric materials at the same time to create plastic articles having a three layer structure. A typical simultaneous injection process might have three phases, the first injects a first polymeric material, the second injects a first and second polymeric material, and the third injects only the first polymeric material. For a simultaneous injection process, the quantity of polymeric material in the first phase determines the location of the leading edge of the second polymeric material into a cavity. During the second phase the relative volumetric flow rates of the first and second polymeric materials determine the thickness of the second polymeric material. When creating plastic articles on a multi-cavity system it is important to control the polymeric material flow entering each cavity so that each cavity creates a nearly identical article. This is particularly true for the first and second polymeric materials to enter the cavity during the first and second phases of a simultaneous injection process.
Those skilled in the art understand that nozzle design interacts with injection process and can also appreciate that other injection processes are possible but each requires control over at least some aspects of the first and second polymeric materials in order to create a multilayer plastic article with the correct layer position and thickness.
Currently, injection systems exist that use shooting pots to control the flow of a first and second polymeric material, but they are too large to meet current industry standards for cavity density. Other injection systems also exist that use thermal balancing on the first polymeric material but not for the second polymeric material.
While shooting pots do precisely meter and feed mold cavities with polymeric materials, a further drawback to their use is they are relatively large pieces of equipment to incorporate into an injection mold system and create a space constraint. This space constraint has been a limitation to making preform molds having the same size and configuration as the most compact molds used to make monolayer preforms.
Consequently, the larger mold sizes needed for multilayer preform molds using shooting pot technology requires systems larger than those needed by monolayer preform molds. As a result, conventional shooting pot technology multilayer preform molds have only ⅓ the number of cavities of the largest monolayer preform molds.
In order to provide multilayer preforms having a cost competitive with monolayer preforms it is desirable to produce multilayer preform molds in the same size and configuration as monolayer preform molds using shooting pot technology. Thus, for a sequential injection process, a method of accurately controlling the flow rate and volume of the first material into the cavity without using shooting pot technology on the first and second material is desirable in order to achieve cavity-to-cavity variations of the first and second material on the order of +/−6%.
Hence, there exists a need to improve upon the shooting pot technology used to control the injection of a first and second material.